ENGINE

Abstract

[Designation of Document] Abstract [Abstract] [Problem] To provide an engine in which axes of a cooling water discharging hole which is formed in a cylinder head for discharging cooling water and sensor mounting holes which are formed in the cylinder head for mounting a water temperature sensor and an exhaust gas sensor are arranged substantially parallel to each other. [Means for Resolution] In an engine 50 which includes a cylinder head 52 in which an exhaust port 61 which discharges an exhaust gas to the outside, a water jacket 122 to which cooling water is supplied, a cooling water discharging passage 121a which discharges the cooling water from the water jacket 122, an 02 sensor mounting hole 116a which is connected to the exhaust port 61 and in which an 02 sensor 116 which detects an oxygen density of the exhaust gas in the inside of the exhaust port 61 is mounted, and a water temperature sensor mounting hole 119a which is communicated with the water jacket 122 and in which a water temperature sensor 119 which detects a water temperature of the cooling water is mounted are formed, wherein axes of the cooling water discharging passage 121a, the 02 sensor mounting hole 116a and the water temperature sensor mounting hole 119a are arranged substantially parallel to each other. [Selected Drawings] Fig. 5

Full Text

[Designation of Document] Specification [Title of the Invention] ENGINE [Technical Field] [0001]
The present invention relates to an engine which is mounted on a scooter-type vehicle and the like and which allows the vehicle to travel, and more particularly to an engine which arranges axes of a sensor mounting hole and the like formed in a cylinder head substantially in parallel to each other thus enhancing the forming efficiency, [Background Art] [0002]
To reduce harmful substances contained in an exhaust gas, it is necessary to keep a best air-fuel ratio corresponding to an engine condition. Accordingly, a water temperature sensor which measures the temperature of the cooling water which cools a cylinder head and an exhaust gas sensor which measures an amount of oxygen which is contained in the exhaust gas are mounted on the cylinder head. The engine is controlled based on detected values from these sensors, (for example, see Patent Literature 1) [0003]
[Patent Literature 1] JP-A-11-200913 [Disclosure of the Invention] [Problems that The Invention is to solve]

However, axial directions of sensor mounting holes for mounting the respective sensors in the cylinder head and a cooling water discharging hole for discharging the cooling water to the outside are not aligned and hence, there has been a drawback that in forming the respective holes, it is necessary to change an angle of a forming shaft at the time of forming the holes whereby a forming time is delayed and the cost is pushed up. [0005]
The present invention has been made to overcome such problems and it is an object of the present invention to provide an engine in which axes of a cooling water discharging hole which is formed in a cylinder head for discharging a cooling water, and sensor mounting holes which are formed in the cylinder head for mounting a water temperature sensor and an exhaust gas sensor are arranged substantially in parallel to each other. [Means for solving the problem] [0006]
To overcome the above-mentioned problems, an engine according to the present invention includes a cylinder head in which an exhaust port for discharging the exhaust gas to the outside, a water jacket to which cooling water is supplied, a cooling water discharging hole(for example, a cooling water discharging passage 121a described in an embodiment) which

exhausts the cooling water from the water jacket, an exhaust gas sensor mounting hole (for example, an 02 sensor mounting hole 116a described in an embodiment) which is connected to the exhaust port and in which an exhaust gas sensor (for example, an 02 sensor 116 described in an embodiment) which detects an oxygen density of the exhaust gas in the inside of the exhaust port is mounted, and a water temperature sensor mounting hole which is communicated with the water jacket and in which a water temperature sensor which detects a water temperature of the cooling water in the inside of the water jacket is mounted are formed, wherein axes of the cooling water discharging hole and the exhaust gas sensor mounting hole, and the water temperature sensor mounting hole are arranged substantially in parallel to each other. [0007]
Further, it is preferable that the cooling water discharging hole, the exhaust gas sensor mounting hole and the water temperature sensor mounting hole are arranged in a state that the axes of the holes extend sidewardly and upwardly of the vehicle body when the engine according to the present invention is mounted on the vehicle body (for example, vehicle body frame 2 described in an embodiment). [0008]
Further, it is preferable that the engine according to the present invention includes a radiator which performs heat

radiation of the cooling water discharged from the water jacket, and the cooling water discharging hole is arranged in a state that the axis of the cooling water discharging hole extends toward the radiator. [0009]
Still further, it is preferable that a seat surface in which the cooling water discharging hole is formed and a seat surface in which the water temperature sensor mounting hole is formed are arranged on a coplanar surface. [Advantage of the Invention] [0010]
By allowing the engine of the present invention to have the above-mentioned constitution, in forming the cooling water discharging hole and the exhaust gas sensor mounting hole and the water temperature sensor mounting hole in the cylinder head, it is unnecessary to change an angle of a forming shaft at the time of forming the holes and hence, the forming becomes easy and a forming time can be shorted. Further, the mounting of the cooling water pipe in the cooling water discharging hole, and the mounting of the exhaust gas sensor and a water temperature sensor in the sensor mounting holes can be facilitated whereby it is possible to shorten an assembling operation time. [0011]
Here, when an engine according to the present invention

is mounted on a vehicle body, by arranging the cooling water discharging hole, the exhaust gas sensor mounting hole and the water temperature sensor mounting hole in a state that the axes of the holes extend sidewardly and upwardly of the vehicle body, in assembling the engine, the engine can be mounted from the vehicle-body sidewardly direction and hence, the assembling operation can be facilitated. [0012]
Further, by arranging the cooling water discharging hole in a state that the axis of the cooling water discharging hole extends toward the radiator, it is possible to shorten a length of the cooling water pipe which connects the cooling water discharging hole and the radiator. [0013]
Still further, by arranging a seat surface in which the cooling water discharging hole is formed and a seat surface in which the water temperature sensor mounting hole is formed on a coplanar surface, the respective seat surfaces are formed at the same time and hence, a forming cost can be reduced. [Best Mode for Carrying out the Invention] [0014]
Hereinafter, the preferred mode for carrying out the present invention is explained based on the attached drawings. Here, a scooter-type vehicle 1 on which an engine according to the present invention is mounted is explained in conjunction

with Fig. 1. The scooter-type vehicle 1 includes a vehicle body frame 2. The vehicle body frame 2 is constituted of a head pipe 3 which is arranged in front portion of the vehicle body frame 2 and extends in the vertical direction, a down frame 4 which extends below from the head pipe 3, a pair of left and right s ide frames 5 which extend downwardly, rearwardly and, thereafter, in the rearward upper oblique direction from a substantially center portion of the down frame 4, and an arcuate curve frame 7 which is mounted on the side frames 5 by way of a side plate 6. [0015]
A steering shaft not shown in the drawings is mounted on the head pipe 3 in a rotatable manner and a front fork 8 is mounted on a lower end of the steering shaft. A front wheel 9 is supported on a lower end of the front fork 8. On the other hand, a handle post 10 is mounted on an upper end of the steering shaft. Further, a handle 11 is mounted on an upper end of the handle post 10, and the front wheel 9 can be steered by manipulating the handle 11. Further, an upper stay 12 and a lower stay 13 are mounted on the head pipe 3 in a state that the upper stay 12 and the lower stay 13 extend frontwardly from the head pipe 3. A front basket 14 is mounted on front ends of the upper and lower stays 12, 13. [0016]
A fuel tank 15 is mounted on lower portions of a pair of

side frames 5 and between a pair of the side frames 5. A link mounting portion 48 which is formed on a front end of a power unit 16 is mounted on a rear end of the curved frame 7 which is positioned behind the fuel tank 15 in a vertically swingable manner and, at the same time, a rear end of the power unit 16 is mounted on a rear end of either one of the rear side frames 5 by way of a rear cushion unit 17. Here, a fuel pump 18 which includes a tumbling sensor therein is mounted on the fuel tank 15, and the fuel can be supplied to the fuel tank 15 from a fuel supply port (cap) 19 which is mounted on an upper portion of the fuel tank 15. Further, the power unit 16 is constituted of an engine 50, a transmission 90, a throttle body 21, an air cleaner 22, an exhaust pipe 23 which is extended rearwardly from the engine 50, and a muffler 24 which is connected to a rear end of the exhaust pipe 23. A rear wheel 25 is mounted on the rear end of the power unit 16. The rear wheel 25 is driven by the power unit 16 and hence, the scooter-type vehicle 1 obtains power and can travel. [0017]
A luggage box 26 and a grab rail 20 are mounted on the rear portion of the side frame 5. The luggage box 26 has a front portion thereof supported on a front side cross pipe 27 which is mounted on the side frame 5 by way of a front side plate 28, and has a rear portion thereof supported on a gate-type frame 29 mounted on the side frame 5. Further, the grab rail 20 is

supported on a rear portion stay 30 which extends upwardly from the side frame 5. Here, helmets 31, 32 are shown in a state that the helmets 31, 32 are accommodated in the inside of the luggage box 26. Further, a rear portion accommodating box 33 is mounted on a rear portion of the grab rail 20. [0018]
In the scooter-type vehicle 1 having the above-mentioned constitution, a front upper portion of the vehicle body is covered with a front cover 34 and left and right front side covers 35 are mounted on lower portions of side portions of the front cover 34. A turn signal lamp not shown in the drawings is mounted on the front side cover 35. A front inner cover 36 is mounted on the rear portion of the front cover 34, and side portions of step floors 37 on which a driver and a pillion passenger place their foots are covered with side covers and under covers 38. Further, a seat 39 is mounted on the luggage box 26, and a lower periphery of the seat 39 is covered with a body cover and a body side cover 40. Still further, a handle cover 42 which covers a handle 11 is mounted above the front cover 34, and a head lamp 43 is mounted on the handle cover 42. Further, a front fender 44 is mounted in a state that the front fender 44 covers the front wheel 9 from above and a rear fender 45 is mounted in a state that the rear fender 45 covers the rear wheel 25 from above.

Further, a tail lamp 46 is arranged below the grab rail 20 and a buttery 41 is accommodated between the front cover 34 and the front inner cover 36. [0020]
Next, hereinafter, the details of the power unit 16 are explained in conjunction with Fig. 2 to Fig. 5. Here, in this embodiment, an arrow F shown in Fig. 2 indicates the frontward direction, and an arrow U shown in Fig. 3 indicates the upward direction. An engine 50 which forms the power unit 16 is constituted of a cylinder head cover 51, a cylinder head 52, a cylinder block 53, and a crankcase 54. A cylinder sleeve 55 is fixed in a cylinder block 53. A cylinder chamber 55a is formed in a cylinder sleeve 55 in a state that the cylinder chamber 55a is surrounded by the cylinder sleeve 55, and a piston 56 is disposed in the cylinder chamber 55a in a vertically slidable manner. The piston 56 is connected to a crankshaft
58 which is rotatably supported in the inside of the crankcase 54 by way of a connecting rod 57. With a combustion chamber
59 which is defined by the cylinder block 53 (cylinder sleeve 55), the cylinder head 52 and the piston 56, an intake port 60 and an exhaust port 61 are communicated by way of an intake opening and an exhaust opening. Further, with respect to a mushroom-shaped intake valve 62 and a mushroom-shaped exhaust valve 63, one ends thereof are mounted on valve shafts and are supported by retainers, while another ends thereof are biased

by valve springs 64, 65 which are supported on the cylinder head in the direction that the intake valve 62 and the exhaust valve 63 normally close the intake opening and the exhaust opening. [0021]
Still further, a cam shaft 66 which performs the open/close operation of the intake valve 62 and the exhaust valve 63 is rotatably arranged in the cylinder head 52. The cam shaft 66 includes a cam driven sprocket wheel 67 and the crankshaft 58. A timing chain 69 is wound around a cam driven sprocket wheel 67 which is mounted on the cam shaft 66 and a cam drive sprocket wheel 68 which is mounted on the crankshaft 58. Due to such a constitution, the cam shaft 66 is rotated in conformity with the rotation of the crankshaft 58 and cams 70, 71 which are formed on the cam shaft 66 and pushes down the intake valve 62 and the exhaust valve 63 by way of rocker arms 72, 73 whereby the intake opening and the exhaust opening are opened and closed. Further, an injector 74 is mounted in the intake port 60 and hence, the fuel is atomized in a form of fine particles and is jetted into the intake port 60 from the injector 74. Still Further, an ignition plug 75 is mounted on the cylinder head 52. [0022]
Here, a cam chain tensioner 84 which adjusts the timing of the opening/closing of the intake valve 62 and the exhaust valve 63 using the cams 70, 71 by adjusting a tension of the

timing chain 69 is mounted on a side portion of the cylinder
block 53.
[0023]
In the engine 50 having such a constitution, the air-fuel mixture of air which is cleaned by the air cleaner 22 and fuel which is jetted from the injector 74 is supplied to the combustion chamber 59 from the intake port 60, and the air-fuel mixture is ignited by the ignition plug 75 and burnt after being compressed by the piston 56 and hence, the energy which rotates the crankshaft 58 by way of the piston 56 is generated. Thereafter, the exhaust gas is exhausted to the outside from the exhaust port 61. [0024]
The crankshaft 58 is constituted of a right crankshaft two-split body 58a and a left crankshaft two-split body 58b. The connecting rod 57 is connected to the crankshaft 58 by way of a crank pin 76 which is arranged to join both of the crankshaft two-split bodies 58a, 58b. Further, the crankcase 54 which accommodates the crankshaft 58 is constituted of a right case two-split body 54a and a left case two-split body 54b. One end of the crankshaft 58 (a journal portion of the right crankshaft two-split body 58a) is supported on the right case two-split body 54a by way of a bearing 77, while another end of the crankshaft 58 (a journal portion of the left crankshaft two-split body 58b) is supported on the left case two-split body

54b by way of a bearing 78. Further, an ACG 79 and a radiator fan 85 are arranged at a distal end portion of the journal portion of the right crankshaft two-split body 58a. [0025]
The left side portion of the left case two-split body 54b extends rearwardly and constitutes a portion of a transmission case 80, and a transmission 90 is accommodated in a transmission chamber 82 which is surrounded by a transmission cover 81 mounted on a left side surface of the left case two-split body 54b. The transmission 90 is constituted of a drive pulley 91 which is mounted on a distal end portion of a journal portion of the left crankshaft two-split body 58b which extends in the inside of the transmission chamber 82 and is integrally rotated with the left crankshaft two-split body 58b, a counter shaft 92 which is positioned behind the transmission chamber 80, extends in parallel to the crankshaft 58 and is mounted in a rotatable manner, a driven pulley 93 which is mounted on a substantially center portion of the counter shaft 92 in a relatively rotatable manner with respect to the counter shaft 92, a clutch 94 which is mounted on a left end of the counter shaft 92 and performs the engagement and the disengagement of the driven pulley 93 and the counter shaft 92, and a transmission belt not shown in the drawings which is wound around the drive pulley 91 and the driven pulley 93 and transmits the rotation of the drive pulley 91 to the driven pulley 93.

The drive pulley 91 is constituted of a fixed pulley two-split body 91a which is mounted on the crankshaft 58 in an integrally rotatable manner and a movable pulley two-split body 91b which is movable in the axial direction relative to the fixed pulley two-split body 91a and is integrally rotatable with the crankshaft 58. The transmission belt is sandwiched between the fixed pulley two-split body 91a and the movable pulley two-split body 91b. On the other hand, the driven pulley 93 is constituted of a fixed pulley two-split body 93a which is mounted on the counter shaft 92 in a relatively rotatable manner and a movable pulley two-split body 93b which is movable in the axial direction relative to the fixed pulley two-split body 93a and is rotatable relative to the to the counter shaft 92. The transmission belt is sandwiched between the fixed pulley two-split body 93a and the movable pulley two-split body 93b. Accordingly, by variably setting pulley widths of the drive pulley 91 and the driven pulley 93, winding radii of the transmission belt to the both pulleys 91, 93 can be continuously changed whereby a transmission ratio can be controlled variably (continuously). [0027]
In parallel to the counter shaft 92, an idling shaft 95 and a rear shaft 96 which mounts a rear wheel 25 thereon are provided in a rotatable manner. A rotational driving force of

the counter shaft 92 is transmitted to the rear wheel 25 by way of the counter shaft 92, the idling shaft 95 and a gear train which is mounted on the rear wheel shaft 96. In this manner, an output of the engine 50 is transmitted to the crankshaft 58. Then, after receiving the change-gear operation of the transmission 90, the rotational drive force is transmitted to the rear wheel 25. [0028]
Here, when the above-mentioned power unit 16 is operated, the respective parts generate heat and hence, it is necessary to cool the respective parts. Accordingly, with respect to the cylinder head 52, a water jacket 122 is formed on the cylinder head 52 and the cylinder head 52 is cooled by the cooling water which flows in the inside of the water jacket 122. With respect to the cylinder block 53, a plurality of cooling fins 86 which project outwardly are formed on a side portion of the cylinder block 53. The cooling fins 86 radiate heat generated in the cylinder block 53 . Further, a cooling wind introducing passage 83 which takes outside air in the inside of the transmission chamber 82 is arranged in front of the transmission case 80 in a state that the cooling wind introducing passage 83 extends frontwardly. Due to such a constitution, the transmission case 80 can be cooled. [0029]
Next, cooling water supply device 120 which supplies the

cooling water in the inside of the water jacket 122 mounted on the cylinder head 52 and cools the cylinder head 52 is explained in conjunction with Fig. 6 to Fig. 11. The cooling water supply device 120 is constituted of a cooling water pump 125 which pressurizes and discharges the cooling water, a radiator 129 which collects the cooling water used for cooling the water jacket 122 and irradiates the heat of the cooling water, and a thermostat 130 which, when a temperature of the cooling water indicates a value below given value, returns the cooling water to the water jacket 122 without performing the heat radiation of the cooling water using the radiator 129. [0030]
The cooling water pump 125 is mounted on one end of a pump shaft 132 which extends substantially in parallel to the right crankshaft two-split body 58a. The cooling water pump 125 includes an impeller 131 which pressurizes the cooling water when the pump shaft 132 is rotated. A pump driven sprocket 133 is mounted on another end of the pump shaft 132. Further, between the pump shaft 132 and the right crankshaft two-split body 58a, a pump idling shaft 134 which extends substantially in parallel to these shafts 58a and 132 is provided. On the pump idling shaft 134, a pump driven gear 136 which is meshed with a pump drive gear 13 5 joined to the right crankshaft two-split body 58a is mounted. Further, a pump drive sprocket 137 is mounted on the pump idling shaft 134 in a juxtaposed manner.

Further, a pump chain 138 is wound around a pump drive sprocket wheel 137 and a pump driven sprocket wheel 133, Due to such a constitution, the rotation of the right crankshaft two-split body 58a which is generated by driving the engine 50 is transmitted to the pump idling shaft 134 by way of the pump drive gear 135 and the pump driven gear 136. The rotation of the pump idling shaft 134 is transmitted to the pump driven sprocket 133 from the pump drive sprocket 137 by way of the pump chain 138 and the pump shaft 132 is rotated. Accordingly, the cooling water is pressurized and is discharged from the discharge opening 126 of the cooling water pump 125. [0031]
In front of the combustion chamber 59 formed in the cylinder head 52, the water jacket 122 which is a space formed so as to cover the combustion chamber 59 is provided. Further, the cylinder head 52 includes a cooling water discharge portion 121 on an upper side portion of the water jacket 122. In the cooling water discharge portion 121, a cooling water discharging passage 121a which is communicated with the outside and the water jacket 122 and discharges cooling water to the outside is formed. Further, the cylinder head 52 includes a cooling water introducing space 123 which is formed below and behind the water jacket 122 and also adjacent to the water jacket 122. The cooling water introducing space 123 opens at a rear side surface of the lower cylinder head 52 and is communicated

tfith the water jacket 122. ;0032]
On the other hand, a cooling water introducing portion L28 which projects sideward is formed on a lower side surface of a front portion of the cylinder block 53. In the cooling water introducing portion 128, a first passage 140a which has an introducing opening 128a which opens downward and extends sideward and a second passage 140b which is communicated with the first passage 14 0a and extends frontward are formed (hereinafter, the first and second passages 140a, 140b are referred to as "cooling water introducing passage 140" as a general term). The second passage 140b includes a discharge opening 128b which opens at a front end surface of the cylinder block 53, When the cylinder head 52 is mounted on the cylinder block 53, an opening portion (discharge opening 128b) of the second passage 140b is communicated with the opening portion of the cooling water introducing space 123 of the cylinder head 52 so that the cooling water introducing passage 140 is communicated with the water jacket 122. Further, a discharge opening 126 of the cooling water pump 125 and the cooling water introducing portion 128 formed in the cylinder block 53 are arranged close to each other and these discharge opening 126 and cooling water introducing portion 128 are connected by a cooling water supply pipe 127.

The thermostat 130 includes a first introducing portion 141 and a second introducing portion 142. The thermostat 130 is configured such that when a temperature of the cooling water which flows in the thermostat 130 from the first and the second introducing portions 141, 142 is equal to or below a given temperature/ the second introducing portion 142 is closed and the cooling water is discharged to a discharge portion 143 from the first introducing portion 141/ while when the temperature of the cooling water is higher than the given temperature, the first introducing portion 141 is closed and the cooling water is discharged to the discharging portion 143 from the second introducing portion 142. Further, a valve element 154 which has an axis thereof inclined frontward in a side view is provided to the inside of the thermostat 130, wherein the inside of the thermostat 130 is partitioned to a first thermostat chamber SI which is communicated with the first introducing portion 141 and a second thermostat chamber S2 which is communicated with the second introducing portion 142 by the valve element 154. [0034]
To the cooling water discharging portion 121 formed on the cylinder head 52, a first cooling water pipe 144 is connected. The first cooling water pipe 144 is bifurcated to a second cooling water pipe 146a and a third cooling water pipe 146b at a branched portion 145. The second cooling water pipe 146a is connected to a suction hole 147 of the radiator 129, while the

third cooling water pipe 146b is connected to the first introducing portion 141 of the thermostat 130. on the other hand, the second introducing portion 142 of the thermostat 130 is connected to a discharging hole 148 of the radiator 129 arranged in the vicinity of the second introducing portion 142 byway of a connecting pipe 149 having rigidity. At the branched potion 145, a positioning portion 145a which is used for performing the positioning when the first to third cooling water pipes 144, 146a, 146b are mounted is formed. [0035]
Here, a hous ing (thermostat cover 130a) which accommodates the thermostat 13 0 therein is integrally formed with a housing which accommodates the cooling water pump 125 and hence, the discharge portion 143 of the thermostat 130 and a suction opening 150 of the cooling water pump 125 are directly connected with each other. Accordingly, the thermostat 13 0 constitutes a portion of a cooling water passage which supplies the cooling water to the suction opening 150 of the cooling water pump 125. By shortening a flow passage through which the cooling water flows, it is possible to allow the thermostat 130 to have a compact shape as a whole. Further, by providing the constitution which enables the connection of the radiator 129 and the thermostat 130 using the connection pipe 149, it is possible to enhance the assembling property of the power unit 16.

According to the cooling water supply device 120 having such a constitution, the cooling water discharged from the cooling water pump 125 flows in the water jacket 122 through the cooling water introducing passage 140 of the cooling water introducing portion 128 formed on the cylinder block 53. Further, the cooling water is discharged to the outside from the cooling water discharge portion 121 after cooling the cylinder head 52. Although the discharged cooling water flows in the suction hole 147 of the radiator 129 or the first introducing portion 141 of the thermostat 13 0, when the temperature of the cooling water is equal to or below a given value, since the second introducing portion 142 is closed by the thermostat 130, the cooling water returns to the cooling water pump 125 without passing through the radiator 129, while the temperature of the cooling water is higher than the given value, the first introducing portion 141 is closed by the thermostat 130 and hence, the cooling water returns to the cooling water pump 125 from the second introducing portion 142 after radiating heat through the radiator 12 9 and circulates. [0037]
In this manner, the intake opening for the cooling water (cooling water introducing portion 128) for cooling the cylinder head 52 is not formed in the cylinder head 52 but is formed in the cylinder block 53 and hence, the degree of freedom

in designing the surrounding of the cylinder head 52 can be enhanced and, at the same time, it is possible to effectively make use of a dead space around the cylinder block 53. Here, as can be clearly understood from Fig. 4 or the like, the cooling water introducing portion 128 is mounted on a front end portion of the cylinder block 53, that is, is arranged in the vicinity of the cylinder head 52 and hence, the cooling water introducing passage 140 can be shortened as a whole whereby the cooling water introducing passage 140 can be light-weighted and, at the same time, a manufacturing cost can be reduced. Further, the forming of the cooling water introducing passage 140 in the cylinder block 53 can be facilitated. [0038]
Here, such a cooling water supply device 120 is arranged in a concentrated manner on a right side portion of the power unit 16, wherein the radiator 129 is accommodated in the radiator cover 152 and the radiator outer cover 153 which are mounted on a right end portion of the right crankcase two-split body 54a. Here, the cooling water discharge portion 121 is formed in a state that the cooling water discharge portion 121 extends toward the suction hole 147 of the radiator 129 from the cylinder head 52 and hence, it is possible to shorten a length of the cooling water pipe (first cooling water pipe 144 and second cooling water pipe 146a) which connects the cooling water discharge portion 121 and the suction hole 147 whereby the

piping of these cooling water pipes can be facilitated. [0039]
Further, by arranging the thermostat 13 0 below the radiator 129, it is possible to make air (bubbles) hardly stay in the thermostat 130. Further, the second thermostat chamber S2 which is communicated with the first introducing portion 142 is arranged below a line Ll which is extended in the horizontal direction from an upper end of the discharge opening 148 and hence, it is possible to make air (bubbles) hardly stay in the second thermostat chamber S2. Here, by inclining an axis L2 of the thermostat 130 with respect to the vertical direction, a height of the thermostat 130 can be shortened and, at the same time, a position where air stays in the inside of the thermostat 130 can be arbitrarily decided. Accordingly, by forming a passage for discharging air at the position, it is possible to perform air bleeding efficiently. [0040]
Here, in the power unit 16 according to this embodiment, to reduce harmful substances contained in an exhaust gas discharged from the engine 50, it is necessary to maintain an air-fuel ratio at an optimum value inconf ormity with a condition of the engine 50. Accordingly, an engine control unit not shown in the drawing controls the injector 74 and the like based on the temperature of the cooling water, concentration of oxygen contained in the exhaust gas and an opening of the throttle and

the like so as to realize the optimum air-fuel ratio. A water temperature sensor 119 which measures the temperature of cooling water is mounted in a water-temperature-sensor mounting hole 119a which is formed in the cylinder head 52 and is communicated with the water jacket 122 and measures the temperature of the cooling water which is discharged from the cooling water discharge portion 121. On the other hand, an 02 sensor 116 which measures the concentration of oxygen in the exhaust gas is mounted in an 02 sensor mounting hole 116a which is formed in the cylinder head 52 and is communicated with the exhaust port 61. Accordingly, the 02 sensor 116 smoothly reaches the activation temperature due to the exhaust gas of high temperature and hence, it is unnecessary to provide the particular heater structure whereby the 02 sensor 116 can be provided at a low cost. [0041]
Here, these water temperature sensor mounting hole 119a and 02 sensor mounting hole 116a are arranged on the side equal to the cooling water supply device 120, that is, on the right side portion of the power unit 16 in a concentrated manner. Further, the axes of the water temperature sensor mounting hole 119a and the 02 sensor mounting hole 116a are arranged to be substantially parallel to the axis of the cooling water discharging passage 121a. Accordingly, in forming the cooling water discharging passage 121a, the water temperature sensor

mounting hole 119a and the 02 sensor mounting hole 116a in the cylinder head 52, it is unnecessary to change an angle of a forming shaft and hence, the forming operation can be easily performed. Further, the mounting operation of the first cooling water pipe 144, the water temperature sensor 119 and the 02 sensor 116 can be performed from one side (right side) of the power unit 16 in a concentrated manner and hence, the assembling can be facilitated. [0042]
Further, as can be clearly understood from Fig. 8 and the like, in the power unit 16 according to this embodiment, the radiator 129 is arranged behind and above the engine 50. Accordingly, the axis of the cooling water discharging passage 121a or the like is arranged to extend upwardly. Here, the exhaust port 61 opens at a lower surface of the cylinder head 52 and the exhaust pipe 23 is arranged below the engine 50 and hence, it is possible to mount the first cooling water pipe 14, the water temperature sensor 119 and the 02 sensor 116 from above whereby the assembling operation can be further facilitated. [0043]
Further, a seat surface on which the cooling water discharge portion 121 is formed and a seat surface on which the water temperature sensor mounting hole 119a is formed are coplanar with the cylinder head 52 and hence, the respective seat surfaces can be simultaneously formed thus reducing a cost

for forming.
[Brief Explanation of the Drawings]
[0044]
[Fig. 1] A side view of a scooter-type vehicle on which an
engine according to the present invention is mounted.
[Fig. 2] A plan view of a power unit.
[Fig. 3] A left side view of the power unit.
[Fig. 4] A right side view of the power unit.
[Fig. 5] A cross-sectional view of a cylinder head.
[Fig. 6] A side cross-sectional view of an essential part
including a cooling water supply device.
[Fig. 7] A plan cross-sectional view of an essential part
including the cooling water supply device.
[Fig. 8] A cross-sectional view of an essential part of the
power unit including the cooling water supply device.
[Fig. 9] A cross-sectional view of an essential part of a
cylinder block.
[Fig. 10] A front view of the power unit.
[Fig. 11 ] A perspective view of the power unit as viewed from
above.
[Description of Reference Numerals and Signs]
[0045]
2: vehicle body frame (vehicle body)
50: engine
52: cylinder head

[Designation of Document] Claims [Claim 1]
An engine including a cylinder head in which an exhaust port which discharges an exhaust gas to the outside, a water jacket to which cooling water is supplied, a cooling water discharging hole which discharges the cooling water from the water jacket, an exhaust gas sensor mounting hole which is connected to the exhaust port and in which an exhaust gas sensor which detects an oxygen density of the exhaust gas in the inside of the exhaust port is mounted, and a water temperature sensor mounting hole which is communicated with the water jacket and in which a water temperature sensor which detects a water temperature of the cooling water in the inside of the water j acket is mounted are formed, wherein
axes of the cooling water discharging hole, the exhaust gas sensor mounting hole and the water temperature sensor mounting hole are arranged substantially parallel to each other. [Claim 2]
An engine according to claim 1, wherein the cooling water discharging hole, the exhaust gas sensor mounting hole and the water temperature sensor mounting hole are arranged in a state that the axes extend sidewardly and upwardly of the vehicle body when the engine is mounted on the vehicle body. [Claim 3]

An engine according to claim 1 or claim 2, wherein the engine includes a radiator which performs heat radiation of the cooling water discharged from the water jacket, and the cooling water discharging hole is arranged in a state that the axis of the cooling water discharging hole extends toward the radiator. [Claim 4]
An engine according to claim 1, wherein a seat surface in which the cooling water discharging hole is formed and a seat surface in which the water temperature sensor mounting hole is formed are arranged on a coplanar surface.